In order to improve controllability on the differential pressure between the electrodes in a fuel cell system, conventionally air pressure was supplied as signal pressure to a pressure-regulating valve. For example, Japanese Patent Application Laid-Open No. 2003-68334 discloses a circulation route in which hydrogen gas is depressurized and supplied to the anode of the fuel cell via the hydrogen gas supply line, and the hydrogen off-gas, which is discharged from the fuel cell, is pressurized by the hydrogen pump and thereafter returned to the hydrogen gas supply line in the downstream of the pressure-regulating valve. In this system, the pressure-regulating valve on the hydrogen gas supply line takes the air pressure, which is pressurized by a compressor supplying the air to the cathode, as signal pressure, and thereby regulates the valve opening degree.